Secret communication system



May 7, 1940. w, R. KOCH SECRET COMMUNICATION SYSTEM Filed June 21, i938 s sneet'ssnee't 1 F1 11 f-l7 J 1;: E-j .Sol/HCL' v T of' 7 HUD/0 VOLTAGE T 63 I 5 5 Y 1.9 .5.9 61Y E l I l l mf: 3-9\; 45 EEE-r: I I I MoD. MR 65 23 -f 1'* if: 21- l:- l I l max/NG -l 15 06a /MPuLo'E 67 GEN |||||l||| afrol/P Gnou? GB01/P I'- 1 2 3 *l l l l l HG. 2. :1-

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:Snnentor Win eI/d R. Koch G ttorneg May 7 1940 w. R. KOCH 2.199.634

. SECRET COMMUNICATION SYSTEM Filed June 2l, 1938 3 Sheets-Sheet 2 TINE .DEL/IY NETWORK IMPI/LSE GEN' ww 161: la/o CHEN/VEL Z/VD.

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l .I I I Snom! r GR;UP I GHZOUP GIEL/)L4 Winfield H. Koch/O Mmmm@ May 7, l 940.

w. R. KOCH SECRET COMMUNICATION SYSTEM Filed June 2l, 1938 2ND. DET. I .F MIXER llllll 3 Sheets-Sheet 3 ZND.

Hup /0 :wmv/VEL 3nventor `Zd R.Koch

Win 'i6 (I ttorneg Patented y 7, 19020 Winfield R. Koch. Haddonfeld, N. J., assigner to Radio Corporation of America, a corporation of Delaware appliauonjime 21, 1938,V serial No. 214,866

6 Claims.

My invention relates to secret radiotelephone systems. and more particularly to a communicating system which transmits and receives signals which are unintelligible when received on an ordinary receiver, and which may be readily adapted for use as a secret multichannel communication system.

The importance of maintaining secrecy in certain types of communication systems is well known. Most radio and telephone systems are subject to the disadvantage that the information so transmitted may be intercepted by unauthorized persons. It is, therefore, an object of this invention to provide a communication system which transmits normally unintelligible signals.

It is a further object to provide a communication system which may be used to transmit more than one conversation at the same time, independently but through the same apparatus.

A further object is to provide a transmitter and a receiver, respectively capable of transmitting and receiving a conversation in accordance with this invention.

A still further object of this invention is to provide a secret communication system in which more than one simultaneous conversation may be transmitted and received in complete secrecy.

A further object of this invention is to provide a transmitter whose carrier output peak voltage remains constant during periods of modulation.

A still further object of this invention is to provide a receiver which is capable of detecting the modulating impulses which a.e impressed on a carrier of the type employed in practicing this o invention.

This invention will be better understood from the following description when considered in connection with the accompanying drawings. Its scope is indicated by the appended claims. Similar reference numerals refer to similar parts throughout the several drawings.

Referring to the drawings,

Figure 1 is a schematic drawing of a transmitter which is capable of generating a modulated carrier in accordance with this invention.

Figure 2 illustrates the envelope of three consecutive wave groups of a signal generated by the transmitter shown in Fig. 1,

Figure 3 is a schematic diagram of a receiver for the reception of a signal of the type illustrated in Fig. 2,

Figure 4 is a schematic drawing of an alternative transmitter which is energized by a plurality of separate voice channels,

Figure 5 illustrates the type of signal Yenvelope which results from the use of a transmitter of the type shown in Fig. 4, and

Figure 6 is a schematic diagram of a receiver for separating and making intelligible the separate audio channels which may be transmitted by the device shown in Fig. 4.

Referring to Fig. l, a source of audio voltage l is connected to the input of a push-pull audio frequency coupling transformer 3. The midtap of the secondary of this transformer is connected through a biasing battery 5 to ground. The secondary terminals of this transformer are respectively connected to the control grids 1 and 9 of a pair of pentagrid mixer tubes I I and I3. Each of the tubes II and I3 may have a screen grid which is properly biased by a battery I5. The anode electrodes of the tubes II and I3 are lrespectively connected through a suitable source a resistor 31 which are connected to represent an artificial line circuit. Thus, each impulse which is impressed across the input to this circuit appears at a slightly later time across the successive portions of the delay network.

'I'he intermediate point of the network, which establishes the first delay period, is connected through a suitable biasing battery 39 to a grid 4I in a pentagrid mixer tube I I. The output of the delay network, which establishes the second delay period, is connected through a suitable biasing battery 43 to a grid 45 of a second pentagrid mixer tube I3.

A third pentagrid mixer tube 41 is employed to transmit a timing impulse. The anode electrode of this tube 41 is connected through an anode potential battery 49 to the first terminal of resistor 2|. Its screen grid is properly biased by a battery 5I. The grid 53 of the third pentagrid mixer is not used and may therefore be grounded or biased by means of a battery 55. Grid 51 is connected to the input of the time delay network 25 through a suitable biasing battery 58.

The second terminal of resistor 2I is grounded. The voltage which is developed across this resistor is impressed across the input of a modulator 59 which, in turn, is connected to a radio frequency amplifier 6I, to which an antenna 63 and ground 65 are connected. Engy for driving the radio frequency amplifier 5l is obtained from a local oscillator 61. The modulator 5S, amplifier 6l and oscillator 61 are conventional and need not be shown in detail.

The operation of this transmitter may be explained in the following manner: Each of the pentagrld mixer tubes II, I3 and 41 is biased to plate current cut-olf during the period at which no impulse signal is present on their respective grids. If an audio frequency voltage from source I is impressed in push-pull on the grids of the first two mixer tubes I'I and I3, no corresponding plate current will flow until a biasing impulse is applied to these grids. This biasing potential is supplied by the impulse generator 23 and the delay network 25. The first biasing impulse will place a positive voltage on the grid of mixer tube 41. Since no modulating voltage is applied to this tube, a pulse of short duration and of uniform amplitude will be applied to the modulator. This, in turn, will cause a pulse of carrier frequency energy to be radiated from the antenna 63.

At some appreciable time after this pulse has been applied to the tube 4-1, a similar positive pulse will be impressed upon grid 4l of mixer tube II. This pulse will cause the tube to draw plate current which will correspond to the instantaneous value of the impressed audio frequency voltage. The pulse will finally reach the grid 45 of the mixer tube |3 andwill similarly cause this tube to draw plate current. It is to be noted that the audio frequency voltage is applied in push-pull to the first two mixer tubes. Consequently, when the audio frequency voltage on the grid of one is becoming more positive, it is necessarily becoming more negative on. the other. As a. result, the plate current of the rst mixer tube Il will increase with each successive application of the 'biasing pulse, while the plate current of the second mixer tube I3 will decrease in response to the applied audio voltage.

Fig. 2 illustrates the resultant carrier which is obtained. Three cycles have been illustrated. Each cycle is started by a timing impulse of uniform amplitude and duration which is generated by a mixer tube 41. The first impulse after each of the timing impulses is shown increasing in amplitude While the second impulse after each of the timing impulses is shown decreasing in amplitude in the manner explained above. A line connecting the peak of similar impulses will dene the audio frequency voltage.

The amplitude of the indexing impulse is normally maintained at a. value considerably greater than the maximum which may be reached by the successive impulses. 'It is evident that the peak value of such a carrier wave envelope is constant. Likewise, the average value is constant. Consequently, no intelligible solmd will be reproduced when such a signal is received on an ordinary receiver.

While I have shown a uniform spacing for the various impulses in each group, this is not essential, ior the time delay periods may be made nonuniform. This will, of course, avoid any possibility of accidental timing or synchronizing to the supersonic group frequency. Also the successive output impulses may be separated by making the time delay period greater than the duration of a biasing impulse.

Fig. 3 illustrates an embodiment of a receiver which is suitable for the reception of carrier frequency waves modulated in the manner described above. This receiver includes a mixer 09, a local oscillator 1|. an intermediate frequency amplier 13 and a detector 15. Each of these elements is conventional and need not be described in detail. The output of the detector is connected through a coupling capacitor 11 to the grids 19 and 8| of a pair of mixer tubes 83 and 85. The anode electrodes of these tubes are connected respectively through plate loading resistors 81 and 09 to a suitable source of anode potential such as battery 9| or the like. The anodes are also coupled respectively through a pair of coupling capacitors 93 and 95 to the input of a conventional push-pull audio amplier and output device 91. 'I'he detector output is also coupled to the cathode 99 of a diode rectier |0|. This rectifier is biased by a battery |03 which is of such a value that rectification takes place only during the peaks of the index impulses. Consequently, a voltage of impulse frequency is developed across resistor |05 and impressed upon the grid of a trode amplier |01. The output of this amplifier is coupled to the input of a time delay network |09. This network is identical to the one used in the transmitter illustrated in Fig. l. Its mid-point, or rst delay section, is connected through a suitable biasing battery I|| to the grid I3 of the mixer tube 83. The output or second delay section is connected through a suitable biasing battery ||5 to the grid ||1 of mixer tube 85. The grids |I3 and I I5 of the two mixer tubes 83 and 85 are biased to cut oi the plate-current in the absence of an impulse voltage. Since the applied :impulse voltage is of the same frequency as that utilized in the transmitter and, since the time delay periods are identical, mixer tubes 83 and 85 will be successively made conducting by the successive impulses during the interval at which the impulse was applied to the corresponding tube in the transmitter; that is, tubes 83 and 85 will be successively operable and will reproduce in their plate circuits the audio frequency voltage which was originally applied to the mixer tubes in the transmitter of Fig. l.

While I have shown a transmitter and receiver in which a single audio frequency voltage has been transmitted and received in push-pull, it is possible, by a modification of my invention, to utilize the successive periods of transmission for a plurality of independent simultaneous audio signals. Such a system may utilize the pushpull system just described, or may operate with one impulse period assigned to each separate channel.

Referring to Fig. 4, adevice of this nature is illustrated. For example, three separate and distinct audio channels |2I, |23 and |25 have been provided, which are respectively connected to grids |21, |29 and |3| of three pentagrid mixer tubes |33, |35 and |31, and to ground through three biasing batteries |20, |22 and |24. A supersonic impulse generator 23 is provided as before. Its output is connected to a time delay network |39 which provides three sections of different delay periods for the three separate audio channels. The successive delay sections of the network |39 are connected respectively to the grids |4I, |43 and |45 of the three mixer tubes. Grid bias for these tubes is supplied by a battery |40 which is connected between the low side of the time delay network and ground.

An indexing impulse is provided by means of a fourth pentagrid mixer |41, whose input is connected to the timing impulse generator and whose output is connected in parallel with that of the other tubes.

The modulated carrier wave envelope is of the form illustrated in Fig. 5. 'I'he index impulse is transmitted at regular` intervals and with constant amplitude. Between the successive index impulses three separate impulses occur, each one representing the instantaneous value of the signal from the respective audio channels.

Fig. 6 illustrates the schematic diagram of a receiver designed to separate and translate these impulses. A conventional superheterodyne input system may be employed as shown by mixer 69, local oscillator 1I, intermediate frequency ampliiler 13, and detector 15. 'I'he output of the detector 15, which includes the supersonic impulse frequency, is rectified by a biased rectier IDI. The output voltage, which is developed across resistor |05, is oi the same frequency and wave shape as the indexing or supersonic impulse generated at the transmitter. A time delay network |30 is employed to produce a series of successive impulses which occur in the same time relation as the impulses at the transmitter. Three mixer tubes |49, |5i and |53 are provided for the separation oi' the demodulated carrier into the three separate audio channels. Each of the mixer tubes is biased below plate current cut-oil in the absence of an impulse signal. Therefore, each tube successively becomes operative upon the application of the impulse signal, at the proper time and in the proper sequence, to separate the carrier into the required separate audio channels.

Although I have illustrated my invention by means of a radio transmitter and receiver, it is not necessarily so operated. 'I'he modulating voltage whichis present in the modulator 59 may be sent over land wires or the like, to a suitable receiver. Likewise, while pentagrid mixer tubes are preferable, other types may be utilized. 'Ihey may be operated by superlmposing the impulse bias on the control grid or by operating on the cathode.

When using the system illustrated in Fig. 4, it is desirable to transmit a false signal over any audio channel which is not actually in use in order to confuse the signal which is transmitted by the channels which are in use.

I claim as my invention:

1. In a system for the secret communication of intelligence, the method of operation which includes the steps of generating a series of indexing impulses of supersonic frequency; successively delaying said impulses; generating a signal voltage; obtaining components of said signal voltage which are in phase opposition, utilizing said delayed impulses to control said components which are in phase opposition to thereby obtain a series of separate successive impulses representing respectively an instantaneous value of said signal voltage having one phase and a subsequen-t instantaneous value of said signal voltage having an opposite phase; transmitting said indexing impulse and said successive impulses; receiving said transmitted impulses; separating said indexing impulse from said successive impulses; successively delaying said indexing impulses; and utilizing said successive delayed impulses to reestablish said signalA voltage.

2. In a system of the type described for communication by normally unintelligible signals, a source of indexing impulses, a network for successively delaying said indexing impulse; a source of signal-representing voltages; means for causing the transmission of a rst signal-representing impulse whose amplitude is directly proportional to saidl signal-representing voltage; means for causing the subsequent transmission of a signalrepresenting impulse whose amplitude is inversely proportional to said signal-representing voltage; and means for transmitting said indexing impulse.

3. A device of the character described in claim 2 which is further characterized in that said means for transmitting said impulses includes a radio transmitter.

4. In a system of the type described for communication by normally unintelligible signals, a source of indexing impulses recurring at a supersonic frequency; a network for successively delaying said indexing impulse; a source of signalrepresenting voltages; means for causing the transmission of a ilrst signal-'representing impulse whose amplitude is directly proportional to said signal-representing voltage; means for causing the subsequent transmission of a signalrepresenting impulse whose amplitude is inversely proportional to said'signal-representing voltage; and means for transmitting said indexing impulse.

5. In a system oi' the type described for communication by normally unintelligible signals, a transmitter which includes a source of indexing impulses having a supersonic frequency; a network for successively delaying said impulses; a source of signal-representing voltages; a pair oi mixer tubes, means for impressing said signalrepresenting voltages on the inputs of said tubes in phase opposition; means for connecting the outputs of said tubes in parallel, means for maintaining said tubes normally nonresponsive to said signal-representing voltages; means for causing said tubes to become successively responsive for the duration of each successively delayed indexing impulse whereby alternate impulses are obtained whose amplitudes are respectively directly and inversely proportionalto the instantaneous amplitude oi' said signal-representing voltages; means for transmitting said .indexing impulse; and means for transmitting said alternate impulses.

6. In a system of the character described in .claim 5, a receiver which includes means for separating said indexing impulse from said alternate impulses; a pair of thermionic tubes. means ior impressing said alternate impulses on the input of said tubes; means for successively delaying said indexing impulse;`means for causing said tubes to become alternately responsive to said impulses for the duration of each successively delayed impulse; whereby said signal-representing voltages are reestablished in the out- Dut circuit of said tubes.

WINFIEIDRKOCH. 

